What Is an ADU? Complete Guide to Accessory Dwelling Units (2026)
An ADU — Accessory Dwelling Unit — is a secondary housing unit on a single-family residential lot. It can be a detached backyard cottage, an attached addition to the main home, a basement apartment, or a garage conversion. ADUs have become one of the fastest-growing housing solutions in the United States, particularly in high-cost cities like Seattle where housing costs make traditional development difficult.
This guide answers every common question about ADUs: what they are, what types exist, what they cost, and how they work specifically in the Seattle metro area.
ADU Types: The Four Main Categories
1. Detached ADU (DADU) — Backyard Cottage
A detached ADU is a completely separate structure on the same lot as the primary home. In Seattle, detached ADUs are commonly called backyard cottages or DADUs (Detached Accessory Dwelling Units). They have their own entrance, kitchen, bathroom, and living space. They can be built from the ground up, or sometimes an existing structure like a garage or workshop is converted.
2. Attached ADU
An attached ADU is connected to the main home but has a separate entrance and functions as an independent unit. A finished basement with its own door, a ground-floor in-law suite, or a rear addition configured as a separate unit are all attached ADUs.
3. Garage Conversion ADU
Converting an attached or detached garage into a livable ADU is one of the most cost-effective ways to add a unit. The structure already exists — the work involves insulation, plumbing, electrical upgrades, windows, and finishing to meet habitable space standards.
4. Junior ADU (JADU)
A Junior ADU is a small unit created within the existing footprint of the primary home — typically from a bedroom or portion of the main floor. JADUs are generally limited to 500 square feet and must share a wall with the main home. Not all jurisdictions recognize JADUs as a separate category from attached ADUs.
ADU Rules in Seattle (2026)
Seattle has some of the most ADU-friendly regulations in Washington State. Key rules as of 2026:
- Number allowed: One detached ADU and one attached ADU per residential lot — meaning a single property can have both a backyard cottage and a basement apartment
- Maximum size: 1,000 sq ft or 60% of the primary home floor area, whichever is greater (up to 1,000 sq ft)
- Height: Up to 24 feet for a two-story DADU
- Setbacks: 5 feet from rear and side property lines (reduced from previous 10-foot requirement)
- No owner-occupancy requirement: Both the main home and ADU can be rented simultaneously
- No minimum lot size
- Parking: No replacement parking required when an existing parking space is eliminated to accommodate an ADU
Related reading: Backyard Cottage Design in Seattle | ADU Costs in Seattle 2026
How Much Does an ADU Cost?
ADU costs vary significantly based on type, size, site conditions, and finish quality. In the Seattle market (2026):
- Garage conversion ADU (400-600 sq ft): $120,000 – $220,000
- Attached ADU (basement or addition, 400-700 sq ft): $150,000 – $280,000
- Detached ADU / backyard cottage (400-800 sq ft): $200,000 – $380,000
- Two-story DADU (up to 1,000 sq ft): $280,000 – $500,000+
These figures include design, permitting, and construction but not landscaping, furniture, or utility connection fees. See our complete ADU cost guide for Seattle 2026 for a full breakdown.
What Is the ADU Permitting Process?
All ADUs in Seattle require a building permit. The process:
- Feasibility assessment: Confirm your lot and zoning allow an ADU and determine the maximum size
- Design and permit drawings: An architect prepares the plans — floor plans, elevations, structural drawings, and energy compliance documentation
- City permit application: Submit to the City of Seattle SDCI for plan review (6-10 weeks standard)
- Construction: Build under the permit with required inspections
- Final inspection and certificate of occupancy
Total timeline from start to occupancy: typically 10-16 months for a detached ADU in Seattle.
ADU Return on Investment in Seattle
A well-designed ADU in Seattle generates $1,800 – $3,200 per month in rental income depending on size, location, and finish level. At $2,400/month on a $260,000 investment, the unit pays for itself in approximately 9 years and generates roughly $28,800 annually thereafter — while also increasing the property value.
ADUs also provide flexibility: rental income now, housing for a family member later, or a home office / studio for personal use.
Do You Need an Architect for an ADU?
Yes. Seattle building permits require architectural drawings. An ADU is a complete dwelling unit — it requires structural design, energy compliance calculations, plumbing and electrical coordination, and drawings that a plan reviewer can approve. The architect’s drawings are the foundation of the entire permitting and construction process.
Piper Cole Architects designs ADUs across Seattle, Kirkland, Bellevue, Shoreline, Edmonds, and the wider metro area. We handle feasibility, design, permitting, and construction support as a complete package.
Piper Cole Architects offers a free initial consultation for residential and commercial projects across the Seattle metro area. No obligation — just useful information.